This invention deals generally with electrical current production by means of a temperature differential between electrodes and more specifically with such a device which combines a thermionic energy converter and an alkali metal thermal to electric converter to increase the electrical power available from a single source of heat.
Both thermionic energy converters and alkali metal thermal to electric converters are well understood in the art and their principles of operation need not be described here in great detail.
An alkali metal thermal to electric converter (AMTEC) moves liquid alkali metal ions through a catonic barrier while blocking the electrons of the alkali metal atoms. It is this separation of the electrons from the atoms which produces an electrical voltage. The movement of the liquid alkali metal can be accomplished by an electromagnetic pump or even with no moving parts because the liquid alkali metal can be moved by means of capillary pumping action. Such AMTEC devices typically operate with input temperatures of 1000 to 1100 degrees K, output temperatures of 600 degrees K, and heat flux inputs of 1 to 2 watts per square centimeter. They produce electricity of low voltage and high current. A typical cell can produce 0.4 volts at 300 to 500 amperes at a conversion efficiency, the ratio of thermal power in to the electrical power out, of about 19 percent.
A thermionic energy converter generates electricity from heat by using a sealed vacuum enclosure which includes a collector surface which is cooled and is separated from a heated emitter by an interelectrode space. The emitter is heated to a high temperature which results in electron emission, and the electrons move to the cooler collector, thus generating an electrical voltage between the emitter and the collector. Typical conditions under which such thermionic energy converters operate are input temperatures of 1900-2000 degrees K and output temperatures of 1000 degree K with a heat flux input of 10-20 watts per square centimeter. A typical thermionic energy converter can produce 0.7 volts at 300-500 amperes at a conversion efficiency of about 10 percent.
It is the order of magnitude difference between the operating heat fluxes of these two devices which have, up until now, made their association impossible. Heat flux is the measurement of the concentration of thermal power, that is, the amount of heat flowing through a standard surface area. For the units being used here the standard area is one square centimeter of surface. While the thermionic energy converter input and output heat flux is typically 10 to 20 watts per square centimeter, the heat flux required for an AMTEC input is typically only 1 to 2 watts per square centimeter.
Attempting to directly use the heat leaving the thermionic converter to power an AMTEC would compare, in the common world of thermal experience, to attempting to cook an egg by the direct application of a gas welding torch or even the direct application of a stove burner to the egg. We commonly use a pan to spread the heat and reduce the heat flux applied to the egg. Similarly, while the total heat power and the temperature on the output of a thermionic energy converter might be proper for an AMTEC, the concentration of heat, the thermal flux, of a typical AMTEC cell might require the thermionic energy converter to operate at less than optimum conditions.